I've heard that digital sensors are less "forgiving" of blown highlights than film. Why is this?

There's something called "characteristic curves". How does this relate, in film and in digital?

Can anything be done about it? Is this a significant advantage for film in some situations, or does it just mean that one's shooting style might need to be a little different? (Or, does it even mean that?)

I recall somewhere that blown highlights in digital are far easier to 'reclaim' in post than underexposing... maybe just a FYI so people don't read this and start -2EV stops all their photos?
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nchpmnJun 25 '11 at 10:46

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@Crashdown: you're thinking of a concept called "expose to the right". However, that applies only to information up to the blown highlights, which are definitely not easy to reclaim.
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mattdmJun 25 '11 at 12:37

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Digitally blown highlight is worse than negative film because transition between blown and light areas is quite harsh. Slide film is only slightly better than digital in rendering details in overblown highlights. You don't even need high magnification to see the digital image blowing promptly plain white, while the negative film gives more gradual fading of details and slide film is somewhere in between.

For example, here's wallpaper from my hallway shot with same exposure settings and same lens with digital and negative film. Film is shot at shorter distance to match field of view. Lighting is provided by off-camera flash in manual mode set on a lightstand just off the right side of frame. Lens hood is used to avoid stray light from flash. Flash power was doubled when shooting slide film to compensate for its lower sensitivity.

The white blotch on digital image catches attention and annoys, while the film image is much more like what could be seen with similar side-lighting. Shooting in RAW can help a little, but the white will still clip quite harshly.

Thanks for this comparisation! Did you shoot in RAW or JPEG?
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Simon A. EugsterJul 8 '11 at 12:08

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Thanks. Let me know when you're in Boston and I'll get you that beer. :)
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mattdmJul 11 '11 at 17:07

Not that I'm demanding or anything :), but what would make this the definitive answer is to see the film image pulled back one stop in developing, and the equivalent done in post-processing in digital.
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mattdmJul 12 '11 at 14:05

Film has always had a more nonlinear response than digital, due to the different processes of exciting chemicals to change states, and collecting electrical charge on a solid state device. Another reason is that film contains grains of different sizes which respond differently to light, whereas most digital sensors are homogeneous.

What you ideally want is a gentle rolloff for highlights whereby it takes more and more incoming light to increase the recorded brightness as you approach blow out territory. This makes it substantially more difficult to reach that point and gives you some recourse to recover detail.

Film gives you this roll-off to a much greater extent to digital, which has an approximately linear response which goes brighter brighter brighter blown.

The only thing that can be done about this (other than erring on the side of underexposure) is to have two differently sensitive areas per pixel, giving a non-linear response.

image (c) fujifilm

Fuji executed this concept with their SuperCCD range. Each pixel comprised a small and large photodiodes. When the big photodiode becomes saturated (thus "blown"), the small less sensitive one can carry on recording meaningful data which is used in place of the main photodiode. This gives you the more gradual rolloff you got with film.

image (c) fujifilm

I don't know why this didn't catch on, as apparently the DSLR version was very popular with wedding photographers not wanting to blow out white dresses...

Can you expand a little bit on the linear/non-linear part?
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mattdmJun 24 '11 at 15:42

@Matt Grum, very interesting, I had never heard of that. In response to the linear/non-linear part of your answer, in your opinion is the non-linear response of film more advantageous than being able to see the image and approximate histogram after the capture?
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Dave NelsonJun 24 '11 at 15:58

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This post has some interesting speculation on why the SuperCCD SLRs failed (summary: killed by full-frame in the wedding market, and since that was the primary buyers, that killed the whole thing). The rest of the thread is just ridiculous forum flamewar, unfortunately.
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mattdmJun 28 '11 at 3:30

I would think a lot could be done using a matrix of varied pixels; low-light sensitivity might be improved, for example, by using a matrix of red, green, blue, and untinted pixels; the untinted pixels would be more sensitive than the others, and might "blow" out easily, but they could provide better detail in the dark parts of the picture; the RGB pixels could fill in details in the brighter parts.
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supercatJan 27 at 16:23

The reason for this is the same as digital audio clipping: when the input channel has reached its maximum, it simply cannot go any higher. Unlike analog film photography, where exposure limits are not "hard", digital photography uses numbers to represent the brightness and colors of each part of an image and these numbers can only go up to a hard maximum value (typically 255). Therefore, anything beyond this value cannot be correctly recorded, resulting in a loss of data in highlights. This generally cannot be recovered from because there is no further tonal data that can be extracted from such parts of an image.

From a technical standpoint, clipping in photography and audio are the same problem in two different applications - a numerical value having reached its limit.

(You asked for Digital Sensors, not particularly CMOS) For one, CCD sensors have the blooming effect, which does not happen on film (or CMOS).

Due to the design of CCD sensors, charge can leak from a saturated sensel to its neighbor in an adjacent line. This way, charge leaks from line to line (vertically) and the result is a vertical strip of bright pixels. The longer charge is accumulated, the longer are these strips. An example can be seen in the linked page.

As I understand the definition of blown highlight, it is when there is no data in that part of the image. So that part of the image is completely white with no texture and no detail.

It is common for a specular highlight to be blown out when following that definition, but we do not expect a specular highlight to include any data so that is ok. But a bride's white dress being blown out is not acceptable as we want to see some texture and detail in that case.

I recommend when capturing images to avoid blowing anything out, in fact not having any pure white or pure black is ideal because it will give you the maximum flexibility in post-processing where you can control what gets blown out and what gets reduced to pure black.

Can anything be done about it? Sure, watch the exposure and avoid over and under exposure.

Is film at an advantage? No, because it is much harder to "watch the exposure".

Take a look at photo.stackexchange.com/questions/13411/… for some reasons why 'avoid blowing anything out, in fact not having any pure white or pure black is ideal' isn't always true. Sometimes its just impossible to avoid and sometimes you want to use pure white or pure black to a creative end.
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rfuscaJun 24 '11 at 14:31

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@rfusca That other question isn't really clear about whether it refers to blown highlights in the original capture or blown highlights as a result of post processing, whilst this question is clearly about the capture
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Matt GrumJun 24 '11 at 15:19

@rfusca - I totally agree with you and with that post, but as I said in my answer I recommend capturing without blowing anything out and then using post processing techniques to have more artistic control. If it is necessary to use the image as captured and pure whites were desired then blowing them out during capture is the way to go.
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Dave NelsonJun 24 '11 at 15:56

In general, when a highlight is 'blown', exposure information from that part of the film (or image area, if digitally shot) cannot be recovered.

When shooting film, which is an analog substrate, you could argue that it is practically impossible to accidentally blow your highlights no matter what the exposure there will always be some grains of film that have not reacted to light. Therefore, in principle, you could perform darkroom-fu to recover this signal.

With digital, this is not true, as a photosite's response can saturate. No amount of post-processing can recover this information.